The regenerative Rankine cycle has been used by the electric power industry for over 100 years. Most commonly the working fluid in these cycles is water. The regenerative Rankine cycle takes steam from a steam generator, produces shaft power by expanding the steam in a turbine, and then condenses the expanded steam in a condenser. Heating of the cycle's working fluid occurs in the steam generator, which may be driven by the combustion of fossil fuel Many modern regenerative Rankine cycles employ a reheating of the steam after an initial expansion in a High Pressure (HP) turbine. After reheating in a Reheater heat exchanger, integral to the steam generator, the steam is returned to the cycle for further expansion in an Intermediate Pressure (IP) turbine, followed by expansion in a Low Pressure (LP) turbine; the LP turbine's exhaust is then condensed in a condenser. If an IP turbine is present (accepting steam from a Reheater), its exhaust temperature is commonly higher than the HP turbine's exhaust temperature. The condensate from the condenser, or feedwater, is then routed by pumps through a series of feedwater heaters in which it is re-heated (regenerated). The heating vehicle for the feedwater is extraction steam obtained from the turbine. Feedwater heaters may be of a contact type or a closed type of heat exchanger. A closed type of heater is also termed a surface type heater, this type of heater has a shell-side and a tube-side configuration where, typically, the shell-side contains the heating fluid and the tube-side contains the fluid being heated. With a contact type of heater the extraction steam is directly mixed with feedwater, the heated feedwater/condensed steam being pumped to the next highest pressure heater. With a closed type of heater the extraction steam is contained on the shell-side of the heat exchanger, the feedwater carried within tubes A classical text on the subject of regenerative Rankine cycles used in power plants is by J. Kenneth Salisbury, Steam Turbines and Their Cycles, Robert E. Krieger Publishing Company, Huntington, N.Y., 1950 (reprinted 1974), especially pages 43–93 and 266–273.
For feedwater heaters the heating mechanism involves condensation of turbine extraction steam, its latent heat transferred to the feedwater. Condensing heat transfer is solely dependent on the saturation temperature associated with the extracting steam, and is thus dependent on the extraction pressure delivered by the turbine. Extraction pressures are governed by the turbine's Flow Passing Ability as integrally established by the next downstream nozzle from the point of extraction. The Flow Passing Ability at any point in a steam turbine represents a reduction of Bernoulli's Equation associated with fluid passing through a nozzle (in the case of a turbine, a ring of nozzles forming the inlet to a turbine stage). When nozzles erode their flow area increases, causing, for a given mass flow, a reduction in inlet pressure and thus a reduction in the associated extraction pressure. A degradation in extraction pressure will degrade a feedwater heater's condensing heat transfer mechanism resulting in a lower feedwater temperature.
The typical design practice in North America is to supply the highest pressure feedwater heater its extraction steam from the HP turbine's exhaust. This highest pressure feedwater heater is the last heater the feedwater encounters before returning to the steam generator (it is also termed the “top heater”). The second highest pressure feedwater heater is supplied extraction steam from the IP turbine. The third highest pressure feedwater heater is supplied extraction steam from the next lowest extraction pressure available from the turbine; and so forth. Refer to FIG. 1.
A common design practice in Europe is to supply the top heater its extraction steam from a mid-point IP turbine extraction, and to supply the next to the top feedwater heater its extraction steam from the HP turbine's exhaust. An improved balance of shell-side to tube-side differential exergies is obtained using this design, even through the second highest pressure extraction steam (from the IP turbine) is used to heat the top heater. The shell-side outlet of the top heater is then routed to a third feedwater heater, as is the drain flow from the second heater. With the European design, the third feedwater heater does not receive extraction steam directly from the turbine. There is no known design, including European, which extracts steam from a turbine to directly heat both the top heater and the third feedwater heater, i.e., heaters placed in series along the feedwater path. Refer to FIG. 2. There is no known application where the turbine extraction leading to a top heater in an European design is adjusted to affect the final feedwater temperature. It is known that a single LP turbine extraction may be designed to deliver steam to a plurality of feedwater heaters, but heaters operating, at design, with the same inlet feedwater conditions. For example, if a regenerative Rankine cycle's LP feedwater heaters contain two groups of heaters, say 5A, 6A & 7A, and 5B, 6B & 7B (where 7A & 7B are the lowest pressure heaters both receiving condensate from the condenser, 5A and 5B being the highest pressure heaters), then a single turbine extraction may supply heaters 5A and 5B, another extraction supplying heaters 6A and 6B, and another extraction supplying heaters 7A and 7B. In yet another variation, as favored by the Westinghouse Electric Corporation, it could be that heater 5A is being supplied an extraction which is different than that which heater 5B is being supplied; i.e., asymmetric extractions. However, there is no known design which extracts a single source of steam from an LP turbine and supplies, for example, heaters 5A and 6B, or 6B and 7B, or 7A and 5A, etc. (i.e., heaters placed in series along the feedwater path). In summary, there is no known art which advocates using a single turbine extraction to supply a plurality of feedwater heaters placed in series along the feedwater path.
The High Pressure (HP) turbine's exhaust pressure is controlled by the next downstream turbine's Flow Passing Ability. The next downstream turbine is typically the Intermediate Pressure (IP) turbine. Thus if the nozzles associated with the first stage of an IP turbine erode, the exhaust pressure associated with the HP turbine will degrade, and will thus degrade the associated feedwater heater. In summary: the inlet area of the first nozzles of an IP turbine will control all upstream pressures to the HP turbine: throughout the Reheater heat exchanger, the Reheater piping, and indeed the HP turbine's backpressure (the HP exhaust). When the HP turbine's exhaust is bled to the highest pressure feedwater heater and the IP inlet nozzle area has eroded, extraction pressure to this heater and thus its saturation temperature will degraded, and thus final feedwater temperature will degrade.
Inlet nozzles of IP turbines erode, typically from solid particles trapped in the steam. Traditionally, and especially for the older machine, they go un-repaired for years given that full electrical generation may still be achieved using higher feedwater flows, and with ever increasing consumption of fuel and combustion air. This situation is aggravated if the power plant's over-sight authority (typically a public utility commission or public service commission) allows ever higher fuel costs to be passed onto the electricity customers. However, eventually capacity issues arise from such higher flows. Examples of equipment limitations resultant from such higher flows include: limitations imposed by a combustion air fan; limitations imposed by an induce draft fan controlling combustion gas back-pressures; limitations imposed by a coal mill's capacity; limitations imposed by the capacity of feedwater pumps; limitations imposed by an auxiliary steam turbine driving a feedwater pump; and the like.
Responsible power plant operators are in need of a solution to such a problem. There is no known art which has addressed the issue of IP turbine nozzle erosion at the operational level, when on-line. IP turbine nozzle erosion will degrade the final feedwater temperature on North American steam plants, and will affect system thermal efficiency causing a higher consumption of fuel.